The invention generally relates to methods for forming transparent conductive oxide layers for photovoltaic devices. More particularly, the invention relates to methods for forming crystalline cadmium tin oxide layer by annealing in the absence of cadmium from an external source.
Thin film solar cells or photovoltaic devices typically include a plurality of semiconductor layers disposed on a transparent substrate, wherein one layer serves as a window layer and a second layer serves as an absorber layer. The window layer allows the penetration of solar radiation to the absorber layer, where the optical energy is converted to usable electrical energy. Cadmium telluride/cadmium sulfide (CdTe/CdS) heterojunction-based photovoltaic cells are one such example of thin films solar cells
Typically, a thin layer of transparent conductive oxide (TCO) is deposited between the substrate and the window layer (for example, CdS) to function as a front contact current collector. However conventional TCOs, such as tin oxide, indium tin oxide, and zinc oxide, have high electrical resistivities at thickness necessary for good optical transmission. The use of cadmium tin oxide (CTO) as TCO provides better electrical, optical, and mechanical properties, as well as stability at elevated temperatures. However, CTO/CdS-based thin film solar cells still have challenges, for example, thick CdS films typically result in low device efficiencies whereas thin CdS films lead to reduced open circuit voltage (VOC). In some instances, to achieve high device efficiencies with thin CdS films, a thin layer of a buffer material, such as a tin oxide (SnO2) layer, is intercalated between the cadmium tin oxide (CTO) and the window (CdS) layers.
The typical method used to manufacture a high quality CTO layer includes depositing a layer of amorphous cadmium tin oxide on a substrate, followed by slow thermal annealing of the CTO layer, which is annealed in the presence of a CdS film in close proximity to the surface of the CTO film, to achieve desired transparency and resistivity. The use of expensive CdS for each annealing step may be economically disadvantageous for large-scale manufacturing as the CdS film is not reusable and the cost of the CdS and the glass support makes the process expensive on a large scale. Further, CdS-based annealing of CTO is difficult to implement in a large-scale continuous manufacturing environment, as the process requires assembly and disassembly of plates before and after the annealing steps.
Thus, there is a need to reduce the number of steps for depositing and annealing of CTO layer during manufacturing of photovoltaic devices, resulting in reduced costs and improved manufacturing capability. Further, there is a need to provide cost-effective electrodes and photovoltaic devices manufactured using cadmium tin oxide having the desired electrical and optical properties.